typo 'Displacemnet' should be 'Displacement'

Hi,

I spotted the typo 'Displacemnet' in two lines of [src/science/params/urban_param_mod.F90](https://github.com/MetOffice/jules/blob/main/src/science/params/urban_param_mod.F90):

Line 102 REAL(KIND=real_jlslsm), ALLOCATABLE :: disp_gb(:) ! Displacemnet height

Line 114 REAL(KIND=real_jlslsm), POINTER :: disp_gb(:) ! Displacemnet height

Cheers

I keep reading the code and take notes of the typos below for reference:
- [src/science/surface/get_us.F90](https://github.com/MetOffice/jules/blob/main/src/science/surface/get_us.F90)
- Line 126 ! Calculation of along-canyon wind componenents, should be 'components'
- Line 252 ! Upstream steet, should be 'street'
- Line 277 ! OUT of recirculation region: tranport across a shear layer, should be 'transport'
- Line 306 ! OUT of ventilated region: tranport across a shear layer, should be 'transport'

- [src/science/surface/urbanz0.F90](https://github.com/MetOffice/jules/blob/main/src/science/surface/urbanz0.F90)
- Line 74 ! (normalised reciprcal of friction velocity), should be 'reciprocal'

Start with the comment lines listed in src/science/params/urban_param_mod.F90, src/science/surface/get_us.F90, and src/science/surface/urbanz0.F90. Correct the reported spelling errors in those comments, then review the diff to confirm that only the documented typos were changed.

Written by the indexing model from the issue text.